Production of oxycellulose material



rates tet 3, ll@7,9l7 Patented Nov. 7, 1 961 3,007,917 PRODUCTION OFOXYCELLULOSE MATERIAL Harry Pauling, Reichertshofen, Upper Bavaria,Germany,

assignor, by mesne assignments, to Leon Simon, Washington, DC.

No Drawing. Filed Apr. 2, 1954, Ser. No. 420,751 Claims priority,application Germany Apr. 2, 1953 9 (Ilairns. (Cl. ee-212 The presentinvention relates to a novel process for the preparation of oxycelluloseand to a novel form of oxycellulose so prepared having acidic propertiesand retaining substantially all of the original strength and othercharacteristics of the cellulose from which it is formed.

It has been previously known that when cellulose is treated foroxidation with nitrogen dioxide or nitrogen tetroxide a material solublein dilute alkali and known as oxycellulose may be so prepared. Inaccordance with United States Patent No. 2,232,990 to Yackel et al. ithas been proposed to treat cellulose with gaseous nitrogen dioxide in areaction vessel at relatively low tempertures so as to prepare anoxidized cellulose which was soluble in 2% sodium hydroxide solution andhaving a C equivalent of at least 13%. In accordance with the methodshereinbefore known, as for example as set forth in the Yackel et a1.patent and in other publications, it has not been possible to prepareoxycelluloses from paper or similar material without considerable damageto the paper or to the properties of the material during the course ofthe oxidation process. It was therefore believed that only comparativelysmall quantities could be processed in a batch process, since when usinglarger quantities of material as would be required for large scaletechnical applications, the elimination of the reaction heat andmaintenance of other reaction conditions caused difliculty.

It is known that the principal cause of the difi'iculties is occasionedby the formation of nitric acid which is unavoidable under theconditions of the reaction, since the reaction water liberated from thecellulose during the oxidation forms nitric acid in accordance with thefollowing equation:

Obviously, according to the above reaction, the higher the nitrogendioxide content of the oxidizing gases the stronger the nitric acid thatis formed, so that up to 70% of monohydrate may thus be produced. Thisreaction of the nitrogen dioxide and nitrogen tetroxide with the watercontent of the material takes place immediately upon initiation of theoxidation treatment so that in the usual treatment with nitrogen dioxideand nitrogen tetroxide, at the usual temperature of about 20 C., an acidwith a concentration of about 60 to 70% of nitric acid is formed on andin the oxidized cellulose. It has been previously proposed to avoid thisdifliculty by drying the material prior to treatingthe same withnitrogen dioxide or nitrogen tetroxide, however, it is impossible tovigorously dry paper in this manner since the vigorous orhigh-temperature drying causes damage to the paper.

In accordance with the present invention, the surprising discovery hasbeen made that the various difficulties set forth previously can beavoided by treating a cellulose material such as paper in stages whereinthe first stage the nitrogen dioxide is substantially diluted with aninert gas such as air or superheated steam at a relatively lowtemperature. In this Way it is possible to utilize the heat of formationof the nitric acid which is especially strong at the beginning of theoxidation process for evaporating this nitric acid into the gas currentand avoid to a considerable extent the impregnation of the material withthe formed nitric acid. It has further been discovered that with the useof diluting gas or the like higher temperatures, i.e. of from 45 to 60C. may be used.

ii In accordance with the present invention it has been found to beespecially desirable to treat the cellulose material in the initialstage with an oxidizing gas comprising nitrogen dioxide or nitrogentetroxide in an amount by volume of from 2 to approximately 6.5% of thediluting gas as for example air, and to thereafter treat the cellulosematerial for a further period with slightly more concentrated gas as forexample a gas containing about 10.5% of nitrogen dioxide and the balancea diluting inert gas, and finally to treat the cellulose material in thelast stage with a more concentrated oxidizing gas as for example a gashaving a volume of approximately 38.5% of nitrogen dioxide until thedesired carboxyl content, for example up to 12%, has been produced.

As a result of the stagewise oxidation of the cellulose material it ispossible to produce in the beginning of the process a relatively dilutenitric acid, i.e. nitric acids, of a maximum concentration of from 25 to30%, and to evaporate the nitric acid as it is formed by the heat ofreaction. Although it is important to prevent accumulation of nitricacid, especially if it is highly concentrated on the cellulose material,nitric acid of such low concentration may be considered harmlessinasmuch as the physical properties of the cellulose material are notimpaired thereby.

lt has been further discovered in accordance with the present inventionthat under the conditions of'relatively low nitrogen dioxideconcentration, paper material and cotton may be treated at temperaturesof from 45 to 60 C. without any damage to the physical propertiesthereof. it has been possible to treat in accordance with the process ofthe present invention cellulose material of comparatively thick crosssection since it is possible to oxidize the entire mass with greatuniformity and prevent cornification or other damage to the cellulosematerial. It is within the broad purview of the present invention toeither utilize an inert gas or in the al ernative to utilize a gas underreduced pressure, i.e. the pressure involved during the process beingsuch that it is equivalent to the partial pressure of the nitrogendioxide when using a diluting gas. It is also within the scope of thepresent invention to utilize a combination of reduced pressure anddiluting gas so long as the partial pressure of the nitrogen dioxide isequivalent to that produced in accordance with a process involving, aspreviously set forth, an initial limited volume of nitrogen dioxide todiluting gas followed by, as previously set forth, successivelyincreasing stages of greater nitrogen dioxide concentration. In eithertype of procedure the fact remains that the nitric acid is removedalmost quantitatively from the oxidized material especially in theinitial stages of the reaction.

As previously pointed out the inert gas may be super heated steam,however, the super heated steam should be of such temperature andpressure that the fibers of the material being oxidized will not sufferdamage. Preferably, a moderately super heated steam at a temperature of50 to C. and at a reduced pressure corresponding to this temperatures isutilized. The advantage of utiiizing super heated steam of this type isthat the residual nitric acid retained by the oxidized celluloseestablishes an equilibrium with the super heated steam according to thecomposition of the steam and nitrogen dioxide and nitric acid evaporatesfrom the liquid adhering to the oxycellulose into the super heated steamwithout changing the moisture content of the material undergoingoxidation. It is within the scope of the present invention torecirculate the super heated steam after rectifying the same to removethe weak nitric acid and thereafter super heating the steam again to therequired degree of dryness. If a portion of the steam is condensedduring this rectification it is then replaced by fresh steam and thenitric acid recovered during the rectification can be reused in theprocess. Although nitric acid from any suitable source may be utilizedfor the process of the present invention, it is especially desirable toreuse the nitric acid and nitrogen dioxide by submitting them in aconvenient manner to thermic cleavage into nitroge oxide, oxygen andsteam. The product of the present process is especially suitable forcommercial purposes in the form of paper, fibers, flakes or wool and itis especially suitable for removing alkaline values from gas vapors andliquids. It is, for instance, suitable for absorbing quantitatively theammonia from a gas mixture containing ammonia and hydrogen sulfidewithout removing any of the hydrogen sulfide. The material may also beutilized for purification or alkali removal from various types of gasessuch as coke gases or synthetically produced gases. Since the materialproduced in accordance with the present process is especially strong andretains all of the original strength of the cellulose material fromwhich it is formed, it is possible to wash a gas with acid in order tofree it of alkaline ingredients and to remove the alkali from this washwater by the use of the oxidized material of the present invention. Thematerial can also be utilized as filter bodies or filter paper, and inconnection with its use as a filter it is especially desirable for theuse as filters in cigarettes or other tobacco products. Due to theextremely low de gree of cornification and saccharification of thecellulose material prepared in accordance with the present invention, itmakes an especially desirable filter for the removal of nicotine andtar. The material forms for example with the nicotine of the tobaccosmoke a water insoluble salt so that the nicotine can be removed fromthe tobacco smoke without doing harm to the tobacco flavor. Experimentshave indicated that nicotine removal of more than 70% and considerabletar removal can be carried out with the material of the presentinvention without using any greater amount of filtering surface thanthose customarily used for cigarette filters. The oxycellulose producedin accordance with the present invention has been found soluble indilute alkali, as for example 2% sodium hydroxide, at a rather lowcarboxyl value for example carboxyl values of 6.5%. It can therefore beutilized with great facility for the preparation of alkaline solutionsof cellulose, as for example for the preparation of transparentcellulose films, or for the preparation of artificial filaments by theviscose or other processes. The material, in view of its high strength,is also of special value for pharmaceutical and medicinal purposes sincethis material can be used as a substitute for cat gut or for woundtampons since its high alkaline solubility renders it capable of beingreadily dissolved in the alkaline body fluids.

The following specific examples serve to illustrate but are not intendedto limit the present invention.

Example I One layer of creped cellulose paper and one layer of plaincellulose Wool in commercial sizes were superposed and wound up to forma roll. This material was treated during 40 minutes at a temperature ofabout 45 C. with nitrogen dioxide-air mixture at a concentration of 6.5%per volume N in a conveniently shaped reaction vessel. Thereafter thenitrogen dioxide concentration was increased to 10.5% per volume andtreatment was continued for 20 minutes. Thereafter, the nitrogen dioxideconcentration was increased to 38.5% per volume and the oxidation wascarried out in the course of four hours. At this time the desiredcarboxyl content of had been reached.

The material was dried with air and subsequently washed with distilledwater for quantitative removal of nitric acid. This material could beprocessed without difficulty into cigarette filters using standardmachinery.

4 Example 11 The same material as in Example I was submitted to the samepre-treatment, but was treated with a nitrogen dioxide-air mixture at aconcentration of 38.5 per volume of N0 for three hours only. Thematerial had a carboxyl content of 6.5% and was soluble in 2% sodiumhydroxide.

Example III Pre-purified coke gas which still contained small quantitiesof hydrogen sulfide and ammonia was passed through a glass tube filledwith the oxidized cotton wool of the present invention. Outcoming gaswas completely free of ammonia.

Example IV The gas water which is obtained in the technical wash of cokegas and which apart from ammonia contained also hydrogen sulfide andcarbon dioxide was passed through a filter cylinder consisting ofoxidized cellulose paper and oxidized cellulose wool prepared accordingto Example I. The filtrate was free of ammonia.

Example V (a) Oxidized cotton prepared according to Example I wasprocessed into cigarette filters of usual size and the nicotineabsorption from the tobacco smoke was analyzed. The degree of absorptionwas 70.6%.

(b) Oxidized cotton wool and oxidized paper of Example I were processedinto cigarette filters of usual size and the nicotine absorption of thetobacco smoke was analyzed. The degree of absorption was 71.5

Example V1 04527 grams of oxidized cotton wool prepared according toExample I were pressed into a small glass tube and 54 cc. of an aqueousnicotine solution containing 25.4 mg. of nicotine was allowed to passthrough the tube in the course of 9% minutes without using vacuum. Thecotton wool removed the nicotine quantitatively. The filtrate was freeof nicotine.

Iclaim:

1. A process for the preparation of oxycellulose which comprisestreating cellulose with a mixture of nitrogen dioxide and diluting gasselected from the group consisting of superheated steam and air in aplurality of stages of increasing nitrogen dioxide concentration, theconcentration of the nitrogen dioxide during the first stage oftreatment being between approximately 2% by volume and 6.5 by volume.

2. A process for the preparation of oxycellulose which comprisestreating cellulose with a mixture of nitrogen dioxide and diluting gasselected from the group consisting of superheated steam and air in aplurality of stages of increasing nitrogen dioxide concentration and ata temperature below about 60 C., the concentration of the nitrogendioxide during the first stage of treatment being between approximately2% by volume and 6.5 by volume.

3. The process of claim 1 wherein, at least in part, in place of thediluting gas reduced pressures are utilized.

4. The process of claim 2 wherein, at least in part in place of thediluting gas reduced pressures are utilized.

5. A process for the preparation of oxycellulose which comprisestreating cellulose with a mixture of nitrog n dioxide and diluting gasselected from the group consisting of superheated steam and air in aplurality of stages of increasing nitrogen dioxide concentration, theconcentration of the nitrogen dioxide during the first stage oftreatment being between approximately 2% by volume and 6.5% by volume,and thereafter removing nitric acid from the material by treating thesame with diluting gas.

6. The process of claim 5 wherein the diluting gas used for removing thenitric acid is superheated steam at a reduced pressure.

6 7. The process of claim 1 wherein the cellulose maof treatment isbetween about 45 C. and 60 C. and terial is paper. the cellulosematerial is paper.

8. The process of claim 1 wherein the temperature of References Cited inthe fil of this patent treatment is between about 45 C. and 60 C.

UNITED STATES PATENTS 9. The process of claim 1 wherein the temperature5 2,232,990 Yackel et a1. Feb. 25, 1941

1. A PROCESS FOR THE PREPARATION OF OXYCELLULOSE WHICH COMPRISESTREATING CELLULOSE WITH A MIXTURE OF NITROGEN DIOXIDE AND DILUTING GASSELECTED FROM THE GROUP CONSISTING OF SUPERHEATED STEAM AND AIR IN APLURALITY OF STAGES OF INCREASING NITROGEN DIOXIDE CONCENTRATION, THECONCENTRATION OF THE NITROGEN DIOXIDE DURING THE FIRST STAGE OFTREATMENT BEING BETWEEN APPROXIMATELY 2% BY VOLUME AND 6.5% BY VOLUME.